A most important function of an eye is to detect the presence of light. A more advanced eye in the animal kingdom can distinguish colours, which are just electromagnetic radiation of different wavelengths. A human eye can see wavelengths in the range of 380 to 760nm. This range is called the visible region. We cannot " see " infra-red or ultraviolet.
Primates are the only group of mammals which can see the full range of colour. Other mammals may see only white, grey and black.
Colour vision is still not totally understood. However, the following is the simple version of a working idea called the Trichromatic Theory.
The photocells in our retina, called cones are responsible for our colour vision. There are 3 types of cones, each with a different iodopsin (a photosensitive pigment).
The colour that our brain "sees" comes from the integration of impulses from the three types of cones. The resultant colour depends on the differential stimulation of each, i.e. how many of each kind are stimulated. When only erythrolabe is stimulated, we will "see" red. When chlorolabe and cyanolabe is stimulated equally, we will "see" yellow. What about all three cones being stimulated equally? White.
We cannot see colour in dim light, because the light intensity is not strong enough to stimulate (or bleach) iodopsin. The threshold frequency of cones is high and so require more light for stimulation, unlike the stimulation of rhodopsin in rods which requires weaker light.
Land, a scientist, discovered that the colour we perceive is affected by the colours of other objects in our visual field. That is, red will appear to be of a darker shade if it is surrounded by squares of green, instead of white. See Visual illusions.
Some people have problems in seeing colours. One kind is colour blindness, a genetic defect. Another kind, however, is due to the overuse of drugs or chemicals, e.g. caffeine.
Test if you have colour blindness. Do you see the number in the chart?
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